The present invention relates to a pressure-resistant helical corrugated pipe adapted to be buried in the ground, so that telecommunication cables such as an electric cable and a telephone cable can be installed in such a corrugated pipe for the purpose of protecting such cables. Such a corrugated pipe is also used as a water main pipe, a sewer pipe, or a liquid transport pipe used in a plant or a factory. The present invention also relates to a method of producing such a pressure-resistant helical corrugated pipe.
Among such conventional pressure-resistant helical corrugated pipes, the type of corrugated pipes adapted for underground use have been required to be pressure-resistant in order to withstand a great ground pressure. In a conventional underground pressure-resistant pipe as shown in FIGS. 11 and 12 (see, for example, Japanese Laid-Open Utility Model Application No. 141889/86 filed by the applicant of the present application), a continuous reinforcing belt plate 06 made of a thin metal plate is embedded in a pipe wall 01 and extends over a top portion 02 of the helical corrugation, opposite side wall portions 03 and 04, extending from the top portion 02, and part of a bottom portion 05.
However, in such a helical corrugated pipe, the reinforcing metal belt plate 06, which is embedded in the top portion 02 of the pipe wall 01, the opposite side wall portions 03 and 04 and the bottom portion 05, is a continuous unitary plate, and therefore in order to further increase a pressure-resistant strength to withstand a flattening force, it is necessary either to increase the thickness of the whole of the metal belt plate 06 or to provide thin metal plates in overlapping relation. If the metal belt plate 06 is increased in thickness, the shaping and processing is not easy, and also it is not easy to impart a helical corrugated shape to the metal belt plate when it is helically wound, and as a result the metal belt plate is liable to deformation due to strain. This results in a problem that it is difficult to manufacture the helical corrugated pipe having a good appearance. On the other hand, if the relatively thin metal plates are used in overlapping relation, it has been found that the metal plates slidingly move relative to one another in contact with one another when an external force (particularly external force to bend the pipe) is applied to the pipe after the formation of the pipe. As a result, the edges of the thin metal plates function to cut the covering resin material so as to produce cracks in the covering resin material, thus causing the damage.